Mice placed in the center of an apparatus, the Elevated-Plus Maze, can choose to enter one of two types of alleys (or arms), an open alley or a closed alley, both of which are elevated significantly above the floor. This behavior can be quantified and is assumed to reflect the balance between an animal's drive to explore new areas and the fear (or anxiety) engendered by being in an open relatively more lit area. The Elevated Plus-Maze behavior of a variety of mouse strain was quantified. As has been reported mouse strains can dramatically differ in their number of entries and their amount of time spent in the open compared to the closed alleys. Two strains, A/J and CBA/J, that differed widely in their Elevated Plus-Maze behavior were crossed to form an F1 generation. The Elevated-Plus Maze behavior of this F1 generation was in between that of the two paternal strains and was relatively independent of which of the two strains provided the maternal contribution. The F1 progeny were then randomly mated to produce an F2 generation. Currently, more than 200 animals in the F2 generation have now been tested for their behavior in the elevated plus maze and DNA prepared. The F2 progeny display a broad range of behavior in the Elevated Plus-Maze with some animals behaving as extreme as animals from the original inbred A/J and CBA/J strains.